Plane: add support for generating a PX4 mixer for failsafe

this creates APM/MIXER.MIX which will be used if the FMU dies to
provide manual control over RC
Plane: added OVERRIDE_CHAN parameter

this allows both for testing PX4IO override on the ground, and for
forced manual override while flying, including re-arming after in-air
reboot
RC_Channel: added channel_function()

this is used in the plane mixing code
HAL_PX4: if there are no enabled channels don't send to PX4IO

this makes it possible to disable PWM output to IO to test override
Plane: improved FLAPIN_CHANNEL docs
Plane: make OVERRIDE_CHAN behaviour exactly match FLTMODE6

this provides a convenient "real manual" flight mode
Plane: handle trims and reversals in px4 mixer creation
Plane: limit RC config to 8 channels

this is a limitation of current px4io.c